Question

Why the moleculer mass of polymers is always as an average?

Answer 1

Because polymers consist of many monomers which are rather small carbohydrates. In a polymerisation reaction they react to very long chains. For example a cup of Polystyrol could, in theory, consist of only one very long chain. This is because the chain will grow it runs out of polymers.

Therefore there is no way you could determine how many molecules there are in one mole and it is simpler to just give an average.